Accurately measuring the velocity of an aircraft in flight can be achieved in many different ways, by mechanical, optical or electronic techniques.
In a known optical technique, a laser is used to illuminate a gas and the Doppler frequency shift due to gas flow in the light scattered by the gas molecules is measured using an interferometer. For the measurement of air speed, an ultraviolet laser is usually used because the intensity of backscatter of light by air molecules is much greater at short wavelengths than at long wavelengths.
Systems of this kind have advantages in measuring air-speed in aircraft because the air flow can be measured at a point remote from the measuring instrument, and therefore remote from flow disturbances caused by the aircraft structure. The laser beam can also be pointed in different directions to measure vector components of air velocity. This enables parameters such as angle of attack and side-slip to be measured; quantities which are difficult to measure reliably using Pitot-tube air flow sensors currently used on aircraft.
However, such systems suffer from the disadvantage that output pulse amplitudes from the laser may be highly variable and unpredictable, so a large amount of amplitude noise is likely to be created which reduces the ability of the gas velocity sensor to resolve small variations in air flow velocity.
Furthermore, temperature variations in either the laser or the interferometer may cause false indications of air velocity. These false indications are caused either by a change in the interferometer due to thermal expansion, thus giving an inaccurate measurement, or by changes in the frequencies of the modes of the laser itself.